Generally, a steam turbine rotor is manufactured in a manner of melting and refining raw materials so as to finally obtain a predetermined chemical composition, which are then cast and solidified in a mold, forging a solidified ingot into a shape of the rotor to obtain a rotor forging product, heat-treating the rotor forging product to obtain a rotor blank, machining the rotor blank, and implanting rotor blades in the rotor blank.
Alternatively, a steam turbine rotor may sometimes be manufactured in a manner of melting and refining raw materials as described above, remelting the resulting ingot in an ESR furnace (ESR) by using the ingot as an electrode and then solidifying the same. A resulting ESR ingot is then forged into a rotor forging product, the rotor forging product is heat-treated to obtain a rotor blank, the rotor blank is machined, and rotor blades are implanted in the rotor blank. A main object of performing the ESR is to improve solidification composition, reduce segregation of components, remove impurities, and so on.
Japanese Patent Application Laid-Open Publication No. 6-155001 (Patent Document 1) discloses a technique for manufacturing an integrated high and low pressure turbine rotor by performing an ESR process using a plurality of hollow electrodes having chemical compositions corresponding to chemical compositions of different parts of the steam turbine rotor.
Japanese Patent Application Laid-Open Publication Nos. 2001-50007 and 2001-50002 (Patent Documents 2 and 3) also disclose techniques for manufacturing a high, medium, and low pressure turbine rotor as well as a low pressure turbine rotor by combining partial rotor blanks of different chemical compositions using the ESR process.
Incidentally, in a thermal plant including a steam turbine, attention is paid to techniques for carbon dioxide emission control in terms of global environment protection, and there is a growing need for more efficient power generation. To improve generation efficiency of a steam turbine, it is useful to increase a turbine steam temperature, and a steam temperature of 600° C. or higher has come to be used recently in a thermal power generation plant equipped with a steam turbine. There is a tendency that the steam temperature rises to 650° C., 700° C., or even above 700° C. in the future.
With such increases in the temperature, the steam turbine rotor applied tends to switch to heat-resistant alloys such as Ni-based superalloys having better high-temperature strength than ferritic heat resistant steels (such as 1% Cr—Mo—V steel or 12% Cr steel), which have insufficient high-temperature strength. However, with such heat-resistant alloys, due to limitations of melting facilities, production on the order of ten-odd tons is a limit in terms of product weight. Further, heat-resistant alloys are higher in cost than ferritic heat resistant steels.
Thus, it is also necessary to keep down the cost of the entire steam turbine rotor by minimizing the scope of application of the heat-resistant alloys. Because of this purpose, rather than as a monoblock structure, it is useful to build the steam turbine rotor as a joined structure of a heat-resistant alloy and ferritic heat resistant steel by using the right materials in the right place.
Possible joined structures for the above purpose include a welded joint and bolted joint. The welded joint has many problems to be solved from the viewpoint of rotor design and long-term reliability, including weld defects, welding deformation, and welding residual stress which may occur in the joint. On the other hand, the bolted joint requires a larger rotor wheel interval in the joint than an optimum design interval, resulting in performance degradation of the steam turbine rotor. Further, the bolted joint is not applicable to a drum rotor structure though applicable to a wheel structure.